1. Field of the Invention
This invention relates to an objective lens driving device employed in an optical pickup device adapted for recording and/or reproducing data signals on or from an optical recording medium such as an optical disk.
2. Related Prior Art
There has so far been used an optical pickup device in which a light beam emitted from a built-in light source such as a semiconductor laser element is converged by an objective lens and other optical devices such as a collimator lens or a beam splitter on a signal recording surface of an optical disk as the optical recording medium and the light beam reflected by the signal recording surface is detected by a photo detector to reproduce the data signals previously recorded on the optical disk, or in which desired data signals are recorded by converging a light beam on the signal recording surface.
With this type of optical pickup device, the objective lens for converging the light beam on the signal recording surface of the optical disk is movably supported in two directions orthogonal to each other, that is, in a direction along the optical axis or a focusing direction and in a direction orthogonal to the optical axis, or in a tracking direction.
Referring to FIGS. 1 and 2, this objective lens driving unit has a lens bobbin 104 supported by a supporting member 102 implanted on a base plate or substrate 101 by means of a lens bobbin supporting arm 103. The lens bobbin supporting arm 103 is constituted of a resilient material, such as a synthetic resin, and is formed with first and second hinges 103a, 103b of reduced thicknesses arranged at right angles with each other. Thus the lens bobbin 104 may be displaced or deflected in two directions, that is, in the focusing direction shown by an arrow f and in the tracking direction t shown in FIG. 1 by the lens bobbin supporting arm 103 being resiliently deformed at the hinges 103a, 103b.
To this lens bobbin 104 are attached an objective lens 105, a pair of focusing coils 106 and a pair of tracking coils 107 on the outer periphery of the focusing coils 106.
These coils 106 and 107 are confronted by a pair of magnetic yokes 108, 108 mounted on the substrate 101 and a pair of magnets 109, 109 mounted on these magnetic yokes 108, 108. Each of the magnetic yokes 108, 108 is formed by a pair of upstanding segments 108a, 108b arranged into a U-shaped element and the magnet 109 is attached to the inner lateral side of one 108a of the upstanding segments while the other upstanding segment 108b is introduced into the associated focusing coil 106 in the form of a hollow rectangular member. Each magnet 109 is confronted by the associated tracking coil 107 which is wound in the form of a flat plate and placed on the outer periphery of the associated coil 106.
The coils 106, 107, the magnetic yokes 108 and the magnets 109 make up a magnetic circuit and the driving electric current is supplied to the coils 106, 107 to drive the lens bobbin 104 in the aforementioned two axial directions.
A balancer 110 is mounted on the lens bobbin 104 on the side opposite to the objective lens 105 with respect to the second hinge 103b. By this balancer 110, the position of the center of gravity of the overall movable parts including the lens bobbin 104, object lens 105 and the coils 106, 107 is brought into registry with the hinge 103b to enable optimum driving of the lens bobbin 104 in the aforementioned two directions.
With the optical pickup device, provided with the above described objective lens driving device, the object lens 105 is driven on the basis of output signals of the photo detector to perform the focusing and tracking servo control operations.
The function of the focusing servo control is to cause the light beam irradiated on the signal recording surface to be converged accurately on the signal recording surface at all times even in cases wherein the optical disk is not rotated in a prescribed plane but rotated with undulations. Thus, as long as the focusing servo is in operation, the objective lens 105 is driven so as to be deviated so that the distance beween the lens and the optical disk is always maintained at a predetermined value.
On the other hand, the function of the tracking servo is to cause the beam spot formed by the convergence of the optical beam to accurately trace the recording track formed on the signal recording surface even when the optical disk is rotated with an offset from its center of rotation.
As the object lens supporting member supporting the object lens so that the lens may be deviated in the focusing and the tracking directions, there are so far known a supporting arm section composed of four round bars, as disclosed in our copending Japanese Patent Specification JU1-2452, or a system including a supporting arm section formed by two sets of parallel spring plates arranged in a direction at right angles with each other and a lens bobbin mounting an objective lens and supported for deviation by the supporting arm section.
With the objective lens driving device, provided with such supporting arm section, the objective lens is driven so as to be deviated on the basis of the output signal of the photosensor, by way of the focusing servo and the tracking servo control operations.
The function of the focusing servo control is to cause the light beam irradiated on the signal recording surface to be converged accurately on the signal recording surface at all times even when the optical disk is not rotated in a prescribed plane but rotated with undulations. Thus, as long as the focusing servo is in operation, the objective lens 105 is driven so as to be deviated so that the distance between the lens and the optical disk is always maintained at a predetermined value.
However, difficulties are presented in the objective lens driving device adapted for supporting the objective lens using four supporting arms composed of the round bars, as shown in the Japanese Patent Specification JU1-2452 in connection with manufacture and assembling because each supporting arm must be attached to the lens bobbin supporting the objective lens.
Also a jumper wire must be provided between the lens bobbin and some stationary portion in order to supply the driving current to the tracking coil and the focusing coil provided in the lens bobbin as the movable member.
This jumper wire must be formed by an extremely thin wire, such as a litz wire, which is sufficiently flexible so as to be displaced to follow the lens bobbin without excess load application during movement of the objective lens. Since this litz wire line is thin and has only minimal mechanical strength, it need be handled with caution, so that difficulties are presented in the wire connecting operation.
Hence, in the above cited Japanese Patent Specification JP1-37733, the supporting arm is formed by two sets of parallel spring plates constituted as the printed circuit board and arranged in directions at right angles with each other.
However, in this case, it becomes necessary to provide an intermediate pillar portion interconnecting a first spring plate displacing the objective lens in the longitudinal or focusing direction and a second spring plate displacing the objective lens in the transverse or tracking direction, so that the weight of the device is increased and hence the device may be subjected to vibrations due to partial resonance in addition to those at the resonant frequency which is determined by the weight of the supporting arm and the movable parts. The vibrations due to partial resonance frequently prove to be harmful resonant vibrations against the biaxial driving of the supporting arms.
On the other hand, when driving the lens bobbin in the focusing direction and/or tracking direction, it is desirable that the first or the second spring plate be displaced only in a direction normal to the other spring plate. However, in practice, partial flexures or distortions are produced, such that not only can the objective lens not be displaced responsive to the control driving current in the focusing and tracking directions with high response characteristics, but harmful resonant vibrations are produced due to the partial flexure or distortions.
On the other hand, with the objective lens driving device, shown in FIG. 1, since the lens bobbin supporting arm 103 is formed of synthetic resin, the resin material may be changed in its properties with the prevailing temperature of the objective lens driving device so that the characteristics of the focusing servo or the tracking servo may be delicately affected to render stable focusing and tracking servo control operations unfeasible. In addition, the thicknesses of the first and second hinges are difficult to control, such that fluctuations in these thicknesses give rise to fluctuations in the spring constants of the hinges and hence it becomes difficult to render the properties of the hinges uniform in the case of mass production.
Also, with the objective lens driving unit shown in FIGS. 1 and 2, the magnetic yokes 108, 108, facing the lens bobbin, are formed by bending a sheet metal into the form of a letter U and are placed on the base plate so that the magnets and the coils of the lens bobbin face each other. Since the electro-magnetic force produced by the magnets of the yokes and the servo current supplied to the focusing and tracking coils of the lens bobbin acts at a site spaced away from a set screw 111 fastening the base plate 100, the magnetic yokes 108, 108 cannot be secured sufficiently, such that excess vibrations may be brought about in the magnetic yokes 108, 108 due to the aforementioned electromagnetic force. Above all, inasmuch as the magnetic yokes 108, 108 are upstanding and hence are free at the distal parts, the segment 108b may be subjectd to vibrations as indicated by the broken lines in FIG. 3.
These vibrations tend to affect the follow-up movement of the lens bobbon responsive to the servo signal to give rise to focusing or tracking errors such that it becomes impossible to perform recording and/or reproduction of data signals with sufficient recording and/or reproducing characteristics.